For producing coffee-based drinks semi-automatic or automatic machines are used with a high number of functions and controls. A high level of automation is applied also to the machines for domestic use and not only professional use.
Typically a coffee machine has a brewing unit, a water reservoir, a boiler for the production of hot water to supply to the brewing unit, a coffee container, an electronic control unit and a user interface. By means of the interface, the user can set a series of functions, parameters for producing the drink and other, depending upon the machine model and the performances thereof. In some cases the machine produces exclusively coffee, whereas in others it can produce even other drinks, for example hot water, cappuccino, hot milk or the like. All these functions can be selected, if present, by means of the user interface. This is used also to signal the user possible failures of the machine, alarm conditions or other events which can prevent the machine from operating correctly. In many machines of this type a reservoir of coffee beans and a grinder to grind the coffee beans and to produce a coffee powder which is supplied to the brewing unit. In other machines the reservoir receives directly the coffee powder and this is supplied in doses predetermined and set by the user to the brewing unit.
One of the problem arising in the modern coffee machines is the need of signalling the user the lack of coffee, in particular the lack of coffee beans in a reservoir upstream the grinder.
When the coffee beans contained in the container is not sufficient for preparing a coffee dose, that is to fill up adequately the brewing chamber of the brewing unit, the machine stops by providing an alarm signal to the user, which has to see to fill up again the coffee container.
In some known machines, in order to control the quantity of ground coffee a volumetric system for measuring the powder produced by the grinder is provided. If the quantity of coffee powder required for one dose of drink is not reached in a pre-set grinding time, this is interpreted by the central unit of the machine as absence of coffee beans in the container. The machine stops and the user has to fill up the container with new coffee beans, starting again the machine. As the central unit knows the volume of ground coffee, in this case it is possible performing a second grinding operation to reach the volume of ground coffee necessary to obtain the cup of coffee.
This system has two drawbacks. First of all it is necessary to adopt a system for measuring the quantity of ground coffee, which increases considerably the costs and the size of the machine. Secondly, the volumetric measurer is subjected to failures and thus it represents a critical element in the operation of the machine. Furthermore, when the coffee beans exhaust during the grinding phase, the machine stops and the user has to fill up again the container, when, on the contrary, one would expect to be able to obtain quickly a cup of drink. This makes not very comfortable the use of the machine. In order to avoid the stop of the machine during the grinding and brewing cycle, the user should control the level of the coffee beans in the container each time before starting the machine.
In more modern machines the coffee volumetric measuring has been eliminated and a measured is performed, based upon the revolutions which the grinder has to make to produce the wished quantity of ground coffee, assuming that between the revolutions and the quantity of ground coffee there is a biunivocal relation, a sufficiently precise assumption for the requested goals. However, in this way when the container of coffee beans empties during the grinding of a coffee charge before the grinder has performed the revolutions corresponding to the coffee dose to be ground, the grinder continues to rotate without coffee therefore without producing powder and the machine is stopped after a certain period of time. In other cases, the stop is performed as a function of a signal indicative of the fact that the grinder is running unloaded. In each case, when the grinder has stopped, the central unit cannot recognize the already ground quantity of coffee and therefore it cannot know how much coffee should still be ground after the user has filled up again the container with beans.
Therefore, when the container of coffee beans is filled up again and the machine is started again, the control unit makes it necessary to unload completely the coffee powder produced in the cycle of partial grinding, which has remained incomplete in the phase preceding the stop, and the whole cycle has to be started again. This involves the clear drawback of wasting a partial coffee dose for each interruption due to the exhaustion of the beans in the container.
In order to detect the absence of beans in the container systems of various type have been studied, for example based upon the measuring of an electric parameter of the engine driving the grinder (WO-A-2008/001403), that is a system based upon measuring the vibrations (US-A-2006/0222736).
Systems of optical type have been studied too, measuring the presence of an adequate quantity of coffee beans in the container. These measuring devices have at least an optical barrier with a sender and a transmitter arranged so that the optical beam intersects the volume of the bean container. When the quantity of beans goes below the level of the optical barrier, the optical detector sends an alarm signal communicating to the user the need to restore the level of beans in the container before this is wholly empty.
These optical systems have considerable drawbacks due to the fact that the environment wherein they have to work involves the deposit of coffee powders or debris preventing the used photocells or optical sensors from operating correctly. Furthermore, the system is particularly expensive as it requires two components to be applied onto the container (a sender and a receiver). Furthermore, it is necessary arranging the sensors inside the walls of the container or manufacturing the walls of the container with transparent material which, however, is likely to get dirty or wear out until preventing the sensors from operating correctly. Furthermore, as it is known, the coffee beans can have difficulties in sliding inside the container and form bridges or blocks of beans which remain temporarily still. If this happens under a condition of poor quantity of coffee contained in the container and locking the beam along the path from the sender to the receiver, the device does not operate and it does not send the signal of lack of coffee, even when the quantity of coffee in the container is insufficient for preparing one dose of drink.
WO-A-2007/000195 describes an automatic coffee machine comprising a reservoir of water and a reservoir of coffee beans. The coffee beans are supplied to an underlying coffee grinder which produces the coffee powder with which the drink is produced. Predetermined doses of coffee powder are loaded into the brewing chamber for each brewing cycle. A capacitive sensor is associated to the reservoir of coffee beans, which sensor should be useful to signal to the user the presence of coffee beans and consequently the lack of beans, but it is not able to provide information useful for handling more correctly the content of the reservoir of coffee beans.